The disclosure relates generally to subsea oil and gas wells. More particularly, the disclosure relates to intervention systems and methods for subsea oil and gas wells. Still more particularly, the disclosure relates to intervention systems and methods for subsea oil and gas wells employing a vertical production tree.
Conventionally, subsea wells are built up by installing a primary conductor in the seabed, securing a wellhead to the upper end of the primary conductor and, with a drilling blowout preventer (BOP) stack installed on the wellhead, drilling down through the BOP stack, wellhead, and primary conductor to produce a wellbore while successively installing concentric casing strings that line the wellbore. The casing strings are typically cemented and/or sealed with mechanical seals at their upper ends.
To convert the cased well for production, a production tubing string is run in through the BOP stack, and a tubing hanger at the upper end of the production tubing string is landed in a mating profile in the wellhead or the tubing spool. Thereafter, bores in the tubing hanger are temporarily closed, and the drilling BOP stack is removed. Next, a production tree having a production bore and associated valves is lowered subsea and mounted to the wellhead or tubing spool. The production tree includes a production outlet coupled to a flowline for producing hydrocarbons from the completed well. In particular, hydrocarbon fluids produced from the wellbore flow through the production tubing and production bore of the tubing hanger, through the production outlet of the tree, and through the flowline to a subsea architecture (e.g., manifold, production riser, etc.).
After completion and during production, it is often necessary to access the well to carry out various operations including, but not limited to, managing the production of oil or gas, altering the geometry or overall state of the well, allowing for various diagnostics to be run, etc. Such processes requiring access to the well after completion and production are often referred to as “intervention” operations. To ensure that the well can be sealed-off and isolated in the event of an emergency situation during an intervention, any intervention method includes, among other things the following three safety capabilities/functions—(1) a means for shearing off downhole components within the wellbore of the well; (2) a means for sealing off the wellbore; and (3) a means for disconnecting the intervention system and surface vessel from the wellhead equipment (e.g., the subsea tree, the wellhead, the tubing spool, etc.).
One traditional method of performing an intervention on a subsea well is to remove a production cap from the upper portion of the production tree and to lower and install a BOP stack on top of the production tree. The BOP stack generally includes a plurality of vertically stacked rams (e.g., blind and/or shear rams), an annular blowout preventer, and an emergency disconnect package (EDP) disposed at the upper end of the BOP stack. The annular blowout preventer and rams are actuatable to seal off the flowbore of the BOP stack. Next, a landing string with a pressure containing pipe, a subsea test tree (SSTT), and a tubing hanger running tool (THRT) is lowered from a surface vessel into the central flow bore of the BOP stack until the THRT is received within the upper end of the production tree. A shut-in or shear valve is located within the SSTT. This shear valve is typically a ball valve that is configured to shear off any coiled tubing or wireline extending through the SSTT in the event of an emergency. A flapper valve is also positioned within the SSTT above the shear valve, and functions to further seal off fluid flow to/from the production tree. Still further, a retainer valve is disposed within the SSTT, axially above both the flapper valve and the shear valve, and functions to retain any fluids within the SSTT and landing string therein in the event of an emergency disconnect.
During an intervention, an emergency situation requiring decoupling of the surface vessel from the wellhead equipment may arise. In such cases, the shear valve closes and cuts off coiled tubing and/or wireline extending through the SSTT. Next, the SSTT disconnects from just above the flapper valve (now closed) and the EDP disconnects from the BOP stack, thereby allowing the surface vessel (e.g., a rig) to move away from the well safely (i.e. without causing damage to the well).
While the above described intervention method has been employed with some success, there are several drawbacks. First, since the intervention method requires the use of a large and heavy BOP stack, a drilling rig with a sufficient lifting capacity is required to perform intervention operations. The expense and time required to secure a drilling rig is burdensome, and may result in a decision to forego the intervention entirely. Second, the bending stresses experienced by the wellhead are substantially increased by the added weight of the BOP stack once it is installed on top of the production tree, thereby potentially fatiguing the wellhead. These drawbacks are usually exacerbated as well pressure increases since high pressure wells typically require larger and heavier equipment. Third, if an emergency disconnect is performed during an intervention, the lower portion of the coiled tubing and/or wireline sheared off by the shear valve of the SSTT may not completely fall through the production tree and into the wellbore, thereby potentially forming an obstruction in the main flow bore of the production tree.
In some conventional surface production trees, shearing the components disposed within the wellbore during an emergency can be accomplished through actuation of one of the isolation valves within the production tree itself. However, such isolation valves are generally not specifically designed to shear metal objects, and thus, can experience significant damage if employed to shear objects passing therethrough. Such damage may undesirably inhibit the ability of the isolation valve to effectively seal. In subsea applications, if an isolation valve cannot maintain an effective seal it must be replaced before production operations may continue, and such subsea replacement necessitates the time consuming and costly retrieval of the entire production tree to the sea surface.